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James Kanze 07-31-2013 04:10 PM

Re: Validity of references returned from iterators
On Wednesday, 31 July 2013 06:07:09 UTC+1, Christopher Head wrote:

> I was in the process of writing an STL-style iterator class. I grabbed
> N3337 (as I don’t have access to the actual standard) and looked at the
> requirements for the various iterator types. I couldn’t seem to find,
> anywhere, the rules describing over what period of time pointers and
> references returned from iterators are valid. That is to say, given
> this code:

> template<typename Titer> void f(Titer iter) {
> T &elt = *iter;
> // Do something with iter
> elt.do_something();
> }

> what are the things I am allowed to do in the commented line and expect
> elt to still be a useful reference to the same element it initially
> pointed to?

> For the standard STL containers, of course, the answer is easy: I’m
> allowed to do pretty much anything with iter, because elt points to an
> object in the underlying container which has nothing to do with the
> iterator.

> I did find this sentence in [iterator.requirements.general]:
> “Destruction of an iterator may invalidate pointers and references
> previously obtained from that iterator.”

> That’s all well and good, but I couldn’t find *any* other cases which
> were documented as allowed to invalidate said pointers and references.

> If Titer were istream_iterator, for example, then doing “++iter” would
> be bad: using elt is, if I understand correctly, undefined behaviour in
> that case. Now, I know that istream_iterator is an input iterator and I
> know that input iterators specify that after invoking ++r, any copies
> of the old value of r are no longer required to be dereferenceable. OK,
> but that doesn’t actually say anything about the validity of elt, which
> was initialized back when iter *was* dereferenceable—and iter, the
> iterator from which elt was initialized, certainly hasn’t been
> destroyed, so the sentence I pasted from general requirements doesn’t
> apply!

> My question is thus: what are the rules, and where in the standard are
> they defined?

The simple answer is that there aren't any rules; each iterator
defines its own rules. And they often involve things other than
the iterator: insert or erase on a container can invalidate
iterators, references or pointers into the container, for
example. Destruction or modification of the iterator can
invalidate the reference (but only for InputIterator, if
I recall correctly). And so on. If you design your own
iterator, you set the rules. If you write a template function
which takes an iterator as a template argument, you specify the
minimum rules that iterator has to adhere to, and so on.


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